The present invention relates generally to wireless communications systems and, more particularly, to transitioning wireless users to a new channel in time division multiple access communications systems.
Many digital cellular telephone networks use time division multiple access (xe2x80x9cTDMAxe2x80x9d) channels for dividing communications resources among the terminals using the network. By way of example, digital cellular telephone networks operating under the TIA/EIA-136 standard employ a frequency division multiple access (xe2x80x9cFDMAxe2x80x9d)/TDMA system in which the frequency band is divided into numerous sub-bands. Each of these FDMA sub-bands, in turn, is divided in time using a repeating 20 ms frame structure. Each 20 ms frame is divided into three 6.67 ms communications time slots. Each of these three time slots on a particular sub-band typically operates as a separate xe2x80x9cchannel.xe2x80x9d
Mobile terminals operating in a mobile wireless communications system in many instances make transitions between channels. For instance, when the mobile terminal moves from the coverage area of one base station to another, a transition between channels may be made. The need to transition between channels may occur during periods of communications and/or when the terminal is in standby mode. In unsynchronized communications systems, the mobile terminal must synchronize to the new channel before communications may proceed. Synchronization to the new channel may also be required even if the base stations are synchronized if, for example, the mobile terminals do not have information regarding their propagation delay to the new base station, or if frequency synchronization is required.
Synchronization is typically accomplished by including a synchronization sequence or xe2x80x9csyncwordxe2x80x9d within each of the communications time slots provided on each frequency sub-band. In traditional TDMA systems, a unique syncword is transmitted during each time slot. Thus, for example, under the TIA/EIA-136 protocol, a total of 162 differential quadrature phase shift keyed (xe2x80x9cDQPSKxe2x80x9d) symbols are transmitted in each time slot, the first 14 of which comprise the above-mentioned syncword. As TIA/EIA-136 allows for up to six time slots per frequency sub-band in a time period of 40 ms (when operating in half rate mode), a total of six unique syncwords are specified, one for each of the TDMA time slots. The mobile terminal knows each of these syncwords, and may use these syncwords to acquire synchronization to the channel as described below.
In transitioning to a new channel, the mobile terminal typically tunes to the carrier frequency of the new channel (if necessary) and then searches for the synchronization word associated with the communication slot to which it has been assigned. As the frame length is 20 ms, the period of uncertainty is 20 ms. However, if the mobile terminal searches for all possible syncwords, it is only necessary to search for the syncword over the time period of one slot plus the length of one syncword. Thus, for example, in TIA/EIA-136 systems, the mobile terminal may, instead of searching the whole 20 ms frame for one syncword, only search over an uncertainty period of 176 symbols (i.e., the 162 symbols which comprise the length of one time slot plus the 14 symbols that are the length of one syncword) for all six syncwords. By searching over this 176 symbol period, it is guaranteed that at least one of the six predefined syncwords will be present in the portion of the received signal that is searched.
In some TDMA systems, the system may provide additional information to the mobile terminal that assists in the transition between channels. For instance, in the TIA/EIA-136 system, an optional information element called xe2x80x9cDelta Timexe2x80x9d may be provided to the mobile terminal by its current base station that enables the mobile terminal to reduce the length (in time) of its search window when transitioning to a new channel. In TIA/EIA-136 systems, the Delta Time information is specified as an offset (in half symbols) from the timing on the current channel. The base station to which the mobile terminal is to transition can calculate this offset by listening to communications between the mobile terminal and its current base station to determine the offset in timing between those communications and the slot to which the mobile terminal will be assigned on one of the new base stations"" frequency sub-bands. This offset information is then provided by the new base station through the cellular network infrastructure to the current base station, which passes it on to the mobile terminal.
As noted above, the Delta Time information notifies the mobile terminal as to when it should transmit its communications to the new base station. However, since the mobile terminal may not have accurate information regarding its propagation delay to the new base station, it typically does not know the precise timing of the signals it is to receive from the new base station. However, given the line-of-sight nature of most ground-based wireless TDMA communications systems, the variability in timing which can result from the physical distance of the mobile terminal from the new base station (which effects the propagation delay) is typically quite small, and, thus, based on the Delta Time information the mobile terminal can typically determine a relatively small uncertainty window in which it can expect to find the synchronization word on bursts transmitted by the new base station. In TIA/EIA-136 communications systems, the uncertainty window when Delta Time information is used is specified as 4 symbols before the expected location of the synchronization word and 19 symbols after the expected location. As the synchronization word is 14 symbols long, this means that the entirety of the synchronization word should fall within an uncertainty window that is 37 symbols in length. Thus, in TIA/EIA-136 systems, the xe2x80x9cDelta Timexe2x80x9d information, if accurate, may be used to reduce the length of the uncertainty window from approximately 500 symbols to 37 symbols.
In a TIA/EIA-136 system, the offset between the end of the burst transmitted by the mobile terminal and the beginning of the burst received by the mobile terminal is defined as the sum of (i) the Standard Offset Reference or xe2x80x9cSORxe2x80x9d and (ii) the Timing Advance or xe2x80x9cTA.xe2x80x9d The SOR is defined as the time difference between the end of the burst transmitted by the mobile terminal and the beginning of the burst received by the mobile terminal when the propagation delay between the base station and the mobile terminal is zero. In TIA/EIA-136 systems, the SOR is always set to 45 symbols. The TA, on the other hand, is a variable parameter, that is set to compensate for the propagation delay between the mobile terminal and the base station (which varies depending upon the mobile terminal""s distance from the base station). Each mobile terminal is assigned a TA value such that the base station may ensure that the bursts of all the mobile terminals using the base station are received simultaneously. During transitions where a mobile terminal is xe2x80x9chanded offxe2x80x9d from one base station to another, the current base station typically provides the new base station an estimate of the TA value that the mobile terminal will have to the new base station.
In the TIA/EIA-136 system specification, the following procedure is defined for using Delta Time information to assist in synchronizing with a new channel:
1. The time for the beginning of the burst that is to be transmitted by the mobile terminal on the new channel (xe2x80x9cT0xe2x80x9d) is set as Toldxe2x88x92DT, where Told is the time at which the beginning of a burst was transmitted over the old channel, and where DT is the Delta Time information value (offset value) that is provided to the mobile terminal.
2. The mobile terminal searches for the known synchronization word on a burst received over the new channel during the times T0+SOR+158 symbols and T0+SOR+195 symbols (a 37 symbol period).
3. If the known synchronization word is found within the window searched in step 2:
a. TA is reset as Sync Pointxe2x88x92SORxe2x88x92162 symbolsxe2x88x92T0, where Sync Point is the timing of the beginning of the first symbol of the synchronization word found in the search of step 2.
b. The TA value is examined to determine if it is between 0 and 30 half symbols. If not, it is likely that the Delta Time information is erroneous. In this situation, the TA is set to the value indicated in the handoff message from the previous channel, and the new transmit time is re-evaluated as Sync Pointxe2x88x92SORxe2x88x92162 symbolsxe2x88x92TA.
4. If synchronization is not found within the window searched in step 2:
a. TA is set to the value indicated in the handoff message from the previous channel.
b. The new transmit time is set as T0=Sync Pointxe2x88x92SORxe2x88x92162 symbolsxe2x88x92TA, where Sync Point is the timing of the received burst on the new channel found by searching the entire 20 ms uncertainty window.
The present invention provides methods and systems or efficiently synchronizing to a new channel in a TDMA system.
In embodiments of the present invention, a transceiver acquires synchronization to a channel in a TDMA communications system by identifying a known synchronization word in a burst received at the transceiver over the channel. A first uncertainty window and a second uncertainty window are defined within a burst that is received over the channel and the transceiver may search in these uncertainty windows for the known synchronization word. The first uncertainty window may be smaller than the second uncertainty window. The first uncertainty window is first searched for the known synchronization word. It may then be determined if the known synchronization word has been located within the first uncertainty window. If it has not been, the second uncertainty window is then searched for the known synchronization word.
In further embodiments of the present invention, a wireless transceiver transfers from a first channel to a second channel in a wireless communications system by acquiring an initial estimate of the frame timing associated with signals received at the transceiver over the second channel and an initial estimate of the frequency setting that will synchronize the transceiver with signals received over the second channel. It is then determined if the initial estimate of frame timing is within a specified number of symbols from an expected location, and if the difference between the frequency setting on the transceiver and the initial estimate of the frequency setting that will synchronize the transceiver with signals received over the second channel is within a second specified Limit. Then, one or more signals received over the second channel are demodulated based on the initial estimates of the frame timing and frequency setting if the initial estimate of frame timing is within the specified number of symbols of the expected location, and if the difference between the frequency setting on the transceiver and the initial estimate of the frequency setting that will synchronize the transceiver with signals received over the second channel is within the specified limit. Thereafter, the initial estimate of the frame timing associated with signals received at the transceiver over the second channel and the initial estimate of the frequency setting that will synchronize the transceiver with signals received over the second channel may be adjusted concurrently with demodulation of the signals.